The Dell Poweredge T420 can hold up to 2 hot swappable power supplies or 1 cabled power supply. The spare power supplies come in Platinum efficiency, hot-plug, redundant 495W, 750W, 1100W power supplies or Silver efficiency cabled 550W power supply options. Make sure you are replacing or adding an additional power supply that matches the wattage of the current power supply installed. Always replace with a genuine Dell power supply specifically engineered for the Dell Poweredge T420. If you are receiving the error message Dell Poweredge T420 power required exceeds PSU wattage you should look at installing the largest wattage power supplies. If you are currently using the largest wattage Poweredge T420 power supply you will want to try replacing them. One of the power supplies could be causing the Poweredge T420 to have a wattage error. If we are not showing stock contact us at 952-831-1032 as we most likely have in servers that we can pull the power supply from.